1. Field of the Invention
The present invention relates to mineral-insulated cables. Such cables, generally metal sheathed, are intended for electricity transportation, electric heat tracing, and temperature measurement applications.
2. Description of the Related Art
Metal sheathed mineral-insulated cables are used in the fields of electricity transportation (as power cable), electric heat tracing (as heating cable) and temperature measurement (as thermocouples). These cables are used in places where high temperature and an aggressive medium may exist. The materials of the metal sheath and conductors are varied for different applications and different environment.
The metal sheathed mineral-insulated cable has a metallic outer sheath, at least one inner conductor, and a mineral-based insulation which is usually magnesium oxide (MgO). MgO is preferably used as the insulation material for the metal sheathed mineral-insulated cables because it is stable at high temperatures and inert with the sheath and conductor materials. It provides a high electricity resistance and is also economical.
The metal sheathed mineral-insulated cable is produced by first filling a metal tube (which forms the metal sheath) with the conductor or conductors and the MgO insulator, then drawing down to the required size which is determined by the application parameters of the cable.
One problem with conventional metal sheathed mineral-insulated cable is that the insulation is susceptible to absorption of water and consequent decreased resistivity. The MgO insulator is composed of magnesium oxide crystal particle clumps, and between the particles there are always cavities. These cavities connect with each other and form channels. Water (moisture) could seep in through these micro-channels in the insulation and be absorbed on the particle surfaces. Since water is a conductor, such seepage will degrade the electricity resistance of the insulation or even result in the cable failing. And the more water that seeps in, the lower the electricity resistance of the insulation will be.
Another problem with the metal sheathed mineral-insulated cable is that as temperature increases, electrical resistance of the insulation decreases. Because low resistance will lead to high leakage current, degradation of the electricity resistance will affect the performance of the cable.
Different materials and methods have been explored to prevent water seepage into MgO, and increase the insulation resistance at enhanced temperatures. None has been entirely satisfactory.
Kaolin has been found to give good results.
According to the present invention a metal-sheathed cable with mineral insulation, in particular a cable with MgO insulation, has kaolin added to the insulation to prevent moisture from infiltrating the insulation and decreasing its resistivity. Kaolin also increases the resistivity at high temperatures.
Kaolin is a naturally-existing clay composed mainly of aluminum silicate. Just how kaolin prevents water seepage into MgO is not very clear. One possible mechanism is that, because kaolin is not as hydrophilic as MgO and kaolin powder is finer than MgO, the fine kaolin powder fills the cavities in the MgO, thus preventing water from seeping in. The effect of kaolin on electricity resistance at high temperature is determined by the property of the material itself.
As a general phenomenon, when temperature increases, the resistance of a material decreases. The decreasing behavior is characteristic for each material (indicated by a factor called resistance temperature coefficient). Kaolin has a smaller resistance temperature coefficient, so that its resistivity decreases more slowly as temperature increases than that of MgO, and as a result, kaolin-doped MgO has a higher resistivity at high temperature than pure MgO.